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Comment by iwonderwhy | Hacker News Reader

Interesting analogy. So in effect to someone “watching” from outside our universe anything moving at the speed of light in our context, is moving either faster or slower than the speed of light in their context. Since the ballon and everything inside is expanding it means that either things are moving at the speed of light + speed of expansion, or speed of light - speed of expansion, when watched from outside. Assuming that everything expands along with the balloon. If I understand this correctly, and according to:

“Due to the expansion increasing as distances increase, the distance between two remote galaxies can increase at more than 3×108 m/s, but this does not imply that the galaxies move faster than the speed of light at their present location (which is forbidden by Lorentz covariance).”

But from outside we can move faster than the speed of light? Sorry having trouble getting my head around this.

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Since there's no frame of reference outside the universe, you can't make that analogy.

I think you're getting tripped up on a variation of this idea, e.g. moving a laser pointer across the moon's surface faster than light. https://www.universetoday.com/109147/how-a-laser-appears-to-...

> Since there's no frame of reference outside the universe

That's unknown. The observable universe is all we know for sure and there's evidence that there is both something beyond what we can observe, which conforms to what we know as space and matter, and something beyond that (whatever existed around the big bang locality).

>there's evidence that there is something beyond what we can observe

By definition, there can't be. Evidence implies something we can observe, which would mean it isn't beyond the observable universe.

Then explain the Hubble volume: unless you believe the Earth is at the center of the universe, basic reasoning tells you that there is a sphere beyond which is outside our light cone, and yet outside which the universe still exists.

Are you saying the observable universe is bounded to our Hubble volume?

>basic reasoning tells you that there is a sphere beyond which is outside our light cone, and yet outside which the universe still exists.

Of course, but basic reasoning isn't evidence. Again... "evidence" implies something that can be observed, and something that can be observed, by definition, lies within the observable universe.

>Are you saying the observable universe is bounded to our Hubble volume?

I'm saying the observable universe is bounded to what we can observe, and evidence of something beyond the observable universe is a contradiction in terms.

I don't know how I can possibly make it simpler.

Wouldn’t that be interesting...

great sci fi plot, the earth is the center of the universe because anything out side of our sphere can’t be observed by our consciousness and ‘collapse’ into existence :)

> Evidence implies something we can observe

That's an arbitrary definition. We can't observe our local past events or even non-local events, yet we use them as theories/evidence depending on context (eg Red Shifted galaxies as evidence of expansion). Reasoning about past events would affect other future events is just as much evidence. That's how physics experiments are generally conducted.

I can concede that there's not necessarily anything "outside" the spacetime bubble. The evidence of nothing is not the existing of something.

kind of yes, relatively from an "outside observer" that would be the case.

but if the usual laws of physics apply to that outside "observer" then it will not be able to observe photons from those objects.

the usual assumption is that space is basically infinite but we can only observe our own "observable universe" sphere of it. every point in space has its own universe basically.

see also cosmic light horizon: https://www.thedallasgeek.com/single-post/2018/06/11/Our-Cos... (the difference between regions we can communicate with and regions from where a photon coming toward us will eventually reach us)

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